Some more research:
the exactly same symptoms I am seeing.
Also, I've set openbox to do "focus follows mouse". If I disable that, everything seems to work fine (except my sanity). Hunh.
gave me the idea to try that out, although I don't know in what way it relates to what I am seeing. The referenced ubuntu bug agains nvidia-* is flagged as fixed, although I am clearly still seeing it.
Any tips? Is it a bug, is it a feature?